I found the problem. I will document here in case someone stumbles accross this in the future:
JBPM 5.4 - Project gwt-console-server.war Inside the lib folder there are these 2 jars: wsdl4j.jar and javax.wsdl_1.5.1.v201012040544.jar.
They have the same classes, but inside the second jar, as it is another version, the class doesnt have the method required.
So, I simply removed that jar. I imagine it is a mistake for both of them to be together, but if someone knows a reason and I will fall in another issue further, please let me know. For now, it solves the problem.
Anyway, thanks everyone for the attention, I was struggling for a while with this.
Regards.